It is well known that polymeric materials can be formed into thin, transparent films and optionally coated with various substances such as silica. When this is done, however, it is difficult to tell whether the film has been properly made, or whether it has flaws, especially if the film has multiple layers, particularly when a gas barrier layer is present. Many critical flaws are not visible. Hand calibration of thickness is not feasible. For oxygen barrier films, the standard analytical instrument for measuring oxygen permeability, as described in U.S. Pat. No. 5,107,696, can only detect average permeability over an area typically five square inches.
Various methods of measuring the presence of oxygen are known for use in various systems. U.S. Pat. No. 5,316,949, issued May 31, 1994 to Bull et al., which is incorporated by reference, discloses a new method which is easy to set up and use, and provides quick results. Compared to the available technology, this method is less expensive, and yields an order of magnitude increase in sensitivity. Unlike available technology, this method can spatially resolve the permeability of an object, i.e., detect streaks, cracks, pinholes and other features. Also, this method more closely correlates with the real-world use for which it was originally developed, to test a food wrap, since it uses a food simulant and ambient temperature, pressure, humidity and gas mixtures for test conditions.
The present application relates more specifically to a multilayer device which can be made for mounting and evaluating the oxygen transmission rate and pattern for an object. An oxygen indicator strip and the test object can be vacuum packaged in a composite, multilayer, oxygen barrier, peelable film structure. The peelable film structure is made so that at least a portion of one of the oxygen barrier layers can be easily separated from the rest of the film structure. Where the oxygen barrier layer has been removed, an underlying oxygen permeable layer is exposed. Once the oxygen barrier layer has been removed, the test can proceed.
Various types of packaging for oxygen sensitive substances are known. Vacuum packages for fresh red meat having peelable layers are disclosed in U.S. Pat. No. 4,886,690 issued to Davis et al. Dec. 12, 1989. U.S. Pat. No. 4,889,731 issued to Williams Dec. 26, 1989 discloses pull tabs for use with such packages. U.S. Pat. No. 4,956,212 issued to Bekele Sep. 11, 1990 discloses a means of improving the effectiveness of oxygen barrier layers in such vacuum packages. U.S. Pat. No. 5,346,735 issued Sep. 13, 1994 to Logan et al., which is incorporated herein as if set forth in full, discloses a peelable film structure with enhanced oxygen barrier properties. None of these references disclose analytical methods for the permeation of oxygen through the packages.
Although this application is written in terms of a specific end use, one of ordinary skill in the art will readily recognize that it is a general tool for detecting cracks and pinholes wherever oxygen might be used as an indicator. For example, it could be used to detect flaws in sheets of aluminum foil. In that case, oxygen permeability per se might not be the primary interest, if one is interested in the physical integrity of the foil. Similarly, the integrity of opaque or tortuous path type materials such as ceramics could be tested as well.